Geoscience Reference
In-Depth Information
meters. Reflector antennas provide half-power beam-widths of a few degrees. These systems
usually scan in azimuth and elevation, within a few minutes, allowing the maximum plume
height to be tracked through time, along with the spatial variations of its reflectivity. Most
have a Doppler capability to measure radial wind velocity, that can be used to infer
information on internal velocities of ash clouds and retrieve information on turbulence,
which has seldom been used. New generation radars are dual-polarized, which may further
help to discriminate ash from hydrometeors.
Powerful weather radars, operating continuously at minute-scale acquisition rate and in all
weather, have been used occasionally to track large ash clouds, chiefly since the first radar
observations of Hekla eruption in 1970 and Augustine in 1976, because the information held
in their records is many fold and potentially very useful for risk mitigation. Scan images of
the ash cloud provide reflectivity variations in horizontal and vertical planes. Time
evolutions of its height and lateral spreading can then be retrieved, along with its ascent rise
rate and lateral transport speed. Mass and volume of radar-detected ash, as well as particle
concentrations in the cloud can be estimated provided the grain size distribution can be
constrained (from ash fall or other).
2.2.1.1.2 Observations
Harris et al. (1981), Harris & Rose (1983) and Rose & Kostinski (1994) first collected
observations of ash plumes from Mt. St. Helens in 1980-1982 using 5 cm and 23 cm radar
systems. They tracked the position of the ash cloud of March 19 1982, and estimated its
volume (2000 500 km
3
), the concentration of ash (0.2-0.6 g/m
3
), reflectivity factors of 4-5
mm
6
/m
3
(6-7 dBZ), and the total mass of ash erupted (3-1010
8
kg). For the famous
paroxysmal eruption of May 18 1980, they obtained a mass of 510
11
kg, an ash volume of
0.2 km
3
, and particle concentration of 3-9 g/m
3
, for the ash cloud downwind of Mount St.
Helens, 1.5-2 h after its eruption (horizontal speed 135 km/h). Reflectivity factors found for
these dense (but distal) ash clouds (7-60 mm
6
/m
3
or 8-18 dBZ) are several orders of
magnitude smaller than those for severe weather considered routinely detectable by
airborne weather radar and dangerous for aviation. Eruption-column rise rates and
horizontal drift of ash clouds of Mount Pinatubo, Philippines, in 1991, were also tracked
using two military C-band weather radars 40 km away (Oswalt et al., 1996). During the
second eruption of June 12, 1991, radars indicated an apparent column rise rate in excess of
400 m/s. Radar height measurements were typically 10 to 15 percent lower than ash cloud
heights inferred from satellite temperature analyses. Radar observations also suggested that
higher eruption columns correlated with greater particle size and density within the
column. Using a C-band radar Rose et al. (1995) found that most intense reflections in an ash
cloud of Mount Spurr in 1992 came from particles 2 to 20 mm in diameter and with a total
particle mass concentration of <0.01 to 1 g/m
3
. The radar did not detect distal parts of the
ash cloud, which have an atmospheric residence time of longer than 30 minutes, because the
larger more reflective ash particles drop out. Maki and Doviak (2001) observed ash plumes
of Mount Oyama on Miyake Island, Japan, in 2000, with a 5-cm (C band) radar, and
proposed a method to obtain the time-dependent size distribution of ash particles from the
time dependence of the reflectivity factor. Lacasse et al. (2004) reported observations of the
ash cloud of the Icelandic Hekla volcano in 2000 with a C-band radar at Keflavík
international airport. Reflectivity factors in the range 30 to >60 dBZ characterized the
eruption column above the vent due to the dominant influence of lapilli and ash (tephra) on
